This invention relates to a cylindrical plain bearing for rotatably supporting a rotating shaft.
A plain bearing for rotatably supporting a rotating shaft is formed to a cylindlical shape so as to be fitted on the inner surface of a housing. Frictional wear and seizure of this plain bearing are suppressed by the lubricating effect and cooling effect of the lubricating oil, which are supplied into a clearance between the inner surface of this plain bearing and the rotating shaft, and the noise suppressing effect can also be brought about by the buffer or damper action of the lubricating oil. As the thickness of an oil film to be formed between the inner surface of the plain bearing and the rotating shaft becomes thicker, that is to say, as the amount of the lubricating oil further increases, the above-mentioned suppressing effect produces good results.
In order to increase the amount of the lubricating oil to be retained, it had been made to provide a plain bearing having a number of fine grooves which are circumferentially formed with on the inner surface of the bearing, the fine grooves being parallel with one another and being arranged to extend in the axial direction of the plain bearing, the fine grooves having a depth of several micro meters (.mu.m).
JP-A-59-73620 discloses a plain bearing having a number of fine grooves which are circumferentially formed on the whole inner surface of the bearing, the fine grooves being arranged in the axial direction of the bearing and being inclined at an angle of 0 to about 5 degrees relative to the circumferential direction of the bearing. This arrangement allows a large amount of lubricating oil to be supplied into the fine grooves, thereby promoting the heat dissipation (Prior Art 1).
JP-U-63-53922 discloses a plain bearing having a number of fine grooves which are circumferentially formed on the inner surface of the bearing, the fine grooves being arranged to extend in the axial direction of the plain bearing, thereby having a large proportion of sliding surface area which is obtained by making the fine grooves in the vicinity of both ends of bearing metals in a larger pitch than those of the axial central portion of the bearing metals. With this arrangement, the lowering in the durability of the bearing is intended to be suppressed by reducing the number of the fine grooves in the vicinity of the both ends of the bearing, where the rotating shaft is in partial contact with the bearing, thereby preventing the decrease in the area which is subjected to the axial loading due to the formation of the fine grooves, as much as possible, so as to reduce the specific load (Prior Art 2).
P-U-63-62621 discloses a plain bearing having fine grooves which are circumferentially formed on the inner surface of the bearing, the fine grooves being arranged to extend in the axial direction of the plain bearing, the fine grooves in the vicinity of both ends of bearing metals being arranged in a deeper depth than those of the axial central portion of the bearing metal. In this Prior Art 3, a line connecting the leading ends of the fine grooves (the bottom portions of the fine grooves) is barrel-shaped so that the depth of the fine grooves in the vicinity of the both ends of the bearing is deeper than those of the rest of the bearing. With this arrangement, in contrast with the Prior Art 2, the amount of the lubricating oil in the both ends which are subjected to a high shock loading due to a partial contact of a rotating shaft is increased to thereby suppress the heat generation in the both ends of the bearing metals (Prior Art 3).
In these Prior Arts 1 to 3, every plain bearing has a number of fine grooves which are circumferentially formed on the inner surface of the bearing, the fine grooves being arranged to extend in the axial direction of the plain bearing, each of side wall portions of the fine grooves functioning as a bank in the axial direction of the bearing. Each of the side wall portions may offer a resistance when the lubricating oil outflowing in the axial direction of the bearing, to thereby prevent an inflow pressure from dropping when the lubricating oil flowing into a clearance between the inner surface of the bearing and the rotating shaft and also allow a large amount of lubricating oil to flow through the fine grooves so as to promote the heat dissipation.
However, if such a bearing is used during a long period of time, the fine grooves will be worn out due to the progress of wear and tear. At the same time, the effect of anti-seizure property and wear-resistant property will be lost rapidly. Particularly, in the Prior Art 3, though the fine grooves in the vicinity of the both ends of the bearing are deeply formed, since those fine grooves will wear out preferentially due to a partial contact of the rotating shaft, the other fine grooves will be worn out by wear and tear, almost concurrently with the wear out of the fine grooves in the vicinity of the both ends of the bearing.